The present invention relates to room temperature vulcanizable silicone rubber compositions and more particularly the present invention relates to a titanium chelate catalyst for room temperature vulcanizable silicone rubber compositions.
One component room temperature vulcanizable silicone rubber compositions are well known. Such compositions generally comprise a silanol end-stopped diogranopolysiloxane polymer, an acyloxy cross-linking agent and a tin salt as the catalyst. The composition was stored in one package in substantially anhydrous state. When it is desired to cure the composition the package is broken and upon exposure to atmopsheric moisture, the composition cross-links to form a silicone elastomer. Most recently, in the case of the acyloxy functional one component room temperature vulcanizable silicone rubber composition there has been developed an alkoxy functional one component room temperature vulcanizable silicone rubber composition (hereinafter room temperature vulcanizable will be referred to as RTV). Accordingly, Nitzche et al disclosed in U.S. Pat. No. 3,065,194 a family of silicone rubber compositions comprising a diorganopolysiloxane polymer having silanol end-stopped groups, an alkoxy functional cross-linking agent and a metal salt chelate organo metallic compound as a base that would serve as the cross-linking catalyst. There have been additional disclosures of one component alkoxy functional RTV compositions such as Brown et al U.S. Pat. No. 3,161,614 which shows a prereacted silanol end-stopped diorganopolysiloxane polymer in combination with the cross-linking catalyst. Cooper et al U.S. Pat. No. 3,383,355 disclosed the preparation of an alkoxy terminated linear siloxane polymer having a solid catalyst, e.g. fuller's earth. U.S. Pat. No. 3,499,859 discloses a hydrocarboxy end-blocked diorganopolysiloxane polymer and a metal containing curing catalyst along with boron nitride. Cooper et al U.S. Pat. No. 3,542,901 discloses a mixture of a linear siloxane having di or trifunctional end-blocking units and a linear siloxane having chemically non-functional inert end-blocking units on one end and di or trifunctional end-blocking units at the other end and including catalyst and the cross-linking agent. Brown et al U.S. Pat. No. 3,122,522 discloses organopolysiloxane intermediates containing condensation and condensable cellosolvoxyl with a catalyst. Brown et al U.S. Pat. No. 3,170,894 discloses organopolysiloxane intermediates containing condensable polyhydrocarboxy radicals with a catalyst and Weyenberg, U.S. Pat. No. 3,175,993 discloses organopolysiloxane intermediates end-blocked with alkoxylated silcarbane groups with a catalyst. However, there was a disadvantage with the catalyst for the above alkoxy systems in that the titanium salts that were disclosed tended to gell the composition and also resulted in an undesirable viscosity build-up during mixing and during storage of the composition for long periods of time. Accordingly, Smith and Hamilton U.S. Pat. No. 3,689,454 and U.S. Pat. No. 3,779,986, Weyenberg U.S. Pat. Nos. 3,294,739 and 3,334,067 and Clark et al U.S. Pat. No. 3,719,635 disclose alkoxy curing compositions in which they utilize a titanium ester chelate catalyst instead of the metal salts of the earlier patents wherein such titanium chelate catalyst controls the gellation and undesirable viscosity build-up during mixing and storage of the compositions for long periods of time such as one to two years. It should be noted that the earlier patents, Weyenberg U.S. Pat. Nos. 3,294,739 and 3,334,067 disclose titanium chelate catalysts which were advantageously utilized with one component alkoxy RTV compositions to produce suitable curing compositions. It should be noted that without the titanium chelate catalyst or some type of titanium salt the alkoxy one component RTV compositions cure too slowly. Subsequent to the discovery of Weyenberg and as disclosed in Smith and Hamilton U.S. Pat. Nos. 3,689,454 and 3,779,986 there was developed a titanium chelate catalyst which was substantially different from the earlier Weyenberg catalyst. Not only did the titanium chelate catalyst of those patents have the same advantageous properties as the Weyenberg catalyst but in addition it imparted to the resulting cured silicone elastomer more advantageous physical properties in terms of tensile strength, elongation, tear, and Durometer.
Then there was a further development in the art as disclosed in Beers U.S. Pat. No. 4,100,129. This patent utilizes substantially the same components as in the former Smith and Hamilton patent but by varying the catalyst to cross-linking agent weight ratio in the composition, there was obtained a composition with exceptionally low modulus and with exceptionally superior adhesion to various substrates. It should be noted that such compositions have been commercialized and have proven to be very successful in the commercial construction sealant market.
However, it was noted after these materials had been marketed for some time, that their shelf lives were largely determined by an appearance problem manifasting itself in the formation of various sizes of crystals ranging from fine sand-like to pellet-like particles. These pellets did not affect the performance of the sealant especially when the sealant was to be used for construction purposes although it did give an unsightly appearance, in the surface texture of the sealant. However, when the sealant was pumped through sealant application lines which were terminated with small outlets these nodules would tend to plug up the orifice thus causing malfunction of mixing and application equipment. It was determined through experimentation and analysis of these nodules that the formation of these nodules usually take six to eight months although a significant number of compositions would not contain them. It was determined that the nodules could be prevented from forming by freezer storage of the compositions prior to use. While such storage was suitable for the utilization of industrial sealants, that is sealants that were to be utilized in factories, it was not found to be a suitable method of nodule prevention for sealants that were to be utilized in construction because the construction sealant distribution network does not have freezer storage available. Accordingly, it was decided to find a way to prevent these nodules from forming.
It must be understood that the explanation given below of nodule formation is merely a theory which seems to be substantiated by the facts and at any rate the nodule problem has been solved as will be pointed out below. Accordingly, proceeding along with our explanation of nodule formation there was utilized 1,3 dioxypropane titanium bis ethylacetoacetate, methyltrimethoxy silane cross-linking systems. Both of these liquid materials reacted to form a titanium chelate with a lower degree of solubility in the system. This occurred due to the following reaction: ##STR2## The resulting methylester titanium chelate of formula II is less soluble than that of formula I which is the catalyst that is inserted in the system. The (II) catalyst version complexes with the filler in the sealant which leads to the the formation of nodules. It was decided that there were four possible approaches to preventing such nodule formation and specifically to prevent the formation of the titanium chelate catalyst of formula II above. These approaches were to utilize a solvent, the possible addition of some amounts of ethanol or ethyl silicate to discourage the above reaction, changing to a titanium chelate which could not undergo the reaction as shown above and finally the preparation of a new mixed chelate.
The first approach which was tried was the solvent approach with toluene or acetonitrile. This approach was successful but was discarded for toxicity and sealant shrinkage problems. The second approach retarded the cure and shortened the shelf-life of the sealant and therefore was discarded. The third approach involved the preparation of 1,3-dioxypropane titanium bis-acetylacetonate of the formula, ##STR3## In the laboratory the third approach was promising except that even this material upon standing for six to nineteen months would complex with small amounts of sediment that formed in the production storage tanks as the result of some natural precipitation processes that are bound to occur. It should be noted that the titanium chelate catalyst in (III) above had to be heated to approximately 40.+-.5.degree. C. to keep the titanium chelate catalyst in solution. Accordingly because of the lack of solubility of the titanium chelate catalyst of Formula (III) even this catalyst would come out of solution and react with minor amounts of sediment to form nodules. Accordingly, it was discovered after much experimentation that the best approach and the best titanium chelate catalyst which would have all the foregoing properties of the Smith and Hamilton and Beers patents mentioned previously but which would have prolonged storage of up to four years or more without the formation of nodules was the utilization of a mixed titanium chelate system.
Accordingly, it is one object of the present invention to provide for a titanium chelate catalyst that does not form nodules even upon standing for periods of three or four years or more.
It is another object of the present invention to provide for a titanium chelate catalyst that is inexpensive to make and does not present gellation problems and still does not form nodules in room temperature vulcanizable silicone rubber compositions.
It is still an additional object of the present invention to provide for alkoxy functional one component RTV compositions having a titanium chelate catalyst which composition does not form nodules for periods of three to four years upon storage prior to cure.
It is yet an additional boject of the present invention to provide a process for producing a titanium chelate catalyst which is relatively inexpensive and efficient which titanium chelate will not form nodules in RTV composition for prolonged periods of time.
It is an additional object of the present invention to provide a process for forming a one component RTV composition which has a novel titanium chelate catalyst in it which will prevent the formation of nodules in the composition for periods of three to four years or more upon standing prior to cure.
These and other objects of the present invention are accomplished by means of the disclosure set herein below.